The limits for CCS: gauging CO2 storage potential

Sedimentary basins worldwide offer CCS options, but first their capacity to effectively hold CO2 must be assessed. Map courtesy of CO2CRC, all rights reserved

Varied systems give divergent answers on how much CO2 can be stored underground, so the IEA has been busy helping develop a uniform tool.

2 May 2014

You have to know the storage capacity of rocks deep beneath land or seafloors for carbon capture and storage (CCS) to work at locking away CO2 there. Numerous proposed classification methods for carbon storage resources exist, but none has been uniformly adopted, resulting in inconsistent and often contradictory estimates. In fact, some estimates have found storage capacity for individual countries that exceed other estimates of similar geology and vintage for total global potential.

So the IEA has spent much of last year helping develop a uniform way to calculate CO2 storage potential.

The constraints in determining capacity

Just as fossil fuels still underground are either resources (total potential material) or reserves (the share that can be extracted profitably by current methods), CO2 storage classification systems also divide potential spaces based on technology, cost and certainty. A storage resource is anything useful and potentially available, while the portion of a geologic resource that has economic value now, and is thus a commodity, is a reserve.

But to differentiate between resources and reserves requires a globally accepted and clear definition of CO2 storage potential, and that became an important step in the IEA effort to build a universal tool to assess storage capacity.

CCS requires a porous geologic formation whose properties allow injection, and then indefinite retention, of CO2. The formation’s storage potential is based on the mass of CO2 that can be stored within the subsurface rocks’ pore space.

Many constraints limit determination of that mass. The first is shortcomings in geological knowledge about the subsurface. Engineers also face restrictions related to injection technologies. Economists have to figure out the costs involved, while policy makers must resolve sociopolitical factors, including regulatory limitations and public views of subsurface CO2 storage.

Some constraints result from government policy; one example is minimum depth requirements for CO2 injection. So each jurisdiction or organisation can yield a different estimate based on assumptions about those limits. Then basic initial estimates must account for variables ranging from whether the site is onshore or offshore, whether or not the CO2 will be injected to enhance oil recovery, and which physical and chemical mechanisms and geological features will retain the CO2 at depth.

Another key component of any storage assessment is the storage efficiency – that is, the fraction of accessible pore volume that the CO2 will occupy. Many factors determine this ratio, including the volume of rock contacted by the CO2 plume, how easily the CO2 will move relative to water present within the pore space and how much water the plume will displace.

Turning theory into methodology

To put these considerations into a methodological framework, the IEA joined with representatives from the geological surveys of Australia, Canada, Germany, the Netherlands, the United Kingdom and the United States for workshops to build a transparent and robust assessment of geologic CO2 storage resource throughout the world, across geologic settings, regardless of the amount of available geologic data. The goal was to create a uniform and coherent process, independent of specific policy choices, to allow comparison of storage assessment results.

The first and fundamental concept to be addressed in any storage assessment is the technically availably storage resource (TASR). The TASR answers the question: how much storage resource is there in total? It comprises the pore space that can be reasonably expected to retain CO2 over a long period of time without adverse environmental impact; in this sense it represents an “upper limit”. Since the TASR is not constrained by economic or policy considerations, it gives a better understanding of the trade-offs that are made when developing policies to control access to resources. Because of this, the TASR allows comparison of different countries’ storage endowments.

Further IEA recommendations outline the next steps to develop storage assessments in a straightforward and uniform way, taking into account systematically applicable policy constraints and limited knowledge of deep depths.

Following these recommendations can give a clear picture of how much CO2 can be stored in different jurisdictions and nations – essential knowledge for CCS to fulfil its potential role as a key CO2 emission abatement technology.

This article by Wolf Heidug, a senior analyst in the IEA Carbon Capture and Storage Unit since 2010 and before that general manager for CO2 policy at Shell International, appears in the new issue of IEA Energy: The Journal of the International Energy Agency. The IEA produces IEA Energy, but analysis and views contained in the journal are those of individual IEA analysts and not necessarily those of the IEA Secretariat or IEA member countries, and are not to be construed as advice on any specific issue or situation. Click here to read the new and earlier issues of IEA Energy, and click here to send a request a free subscription.